Task 2: Locating your Position
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We can now move on to the next aspect of compass use. In this example, let's say you are unsure of your exact position. With your compass and a topographic map, you can find your position with a good deal of accuracy in most situations. You will need a map that has been laminated, in order to draw and erase lines on it with a dry-erase marker.
First, you must align the map to True North and, in turn, to the terrain around you. Lay out your map on a flat and level surface, or as close to flat and level as you can find. It is helpful to place the north side of the map generally northwards when laying it out. Open the cover of your compass all the way so it lays flat on the map and align the scale edge with the True North line on the map. If, due to your location, True North aligns with the lines of longitude on the map without error, it's simpler to align the compass scale edge on one of the lines of longitude near the declination guide. Here we aligned the compass with a public land survey line which runs True North (see our How to Read a Topographic Map article for more info). Now, rotate the map (with the compass still on top of it) until the north arrow points in the same exact direction as magnetic north in the declination diagram. In the image below, our declination diagram shows that Magnetic North is 15 degrees east of True North. So, we rotate the map until the North arrow is 15 degrees east of the index line, and the index line is therefore over an azimuth of 345 degrees (345 being North's 0 degrees minus 15).
Check to be sure the compass hasn't been nudged off the line that you aligned it with. If it's still aligned properly, then your map is now aligned to the terrain and the lines of longitude are running to True North.
Next, start looking for two prominent features within your view that also appear on the map. Accurately matching the terrain you're seeing in the real word to what your seeing on the map (called 'reading the terrain') is essential to this process. If you're unsure of a feature, don't use it. These features can be anything from man-made objects to mountaintops, so long as you can positively match them to their location on the map. In this example, we use two hilltops.
Once you've chosen you're terrain features, remove the compass from the map and take a sighting off your first feature to determine its azimuth. Then, lay the compass flat on the map and rotate the compass until the index line shows the same azimuth. While maintaining this azimuth, place one end of the scale edge in the center of the terrain feature on the map. Draw a line along the scale edge. In the image the first feature is a hilltop at an azimuth of 320 degrees.
You will now take a sighting off your second feature and repeat the process. Here, our second feature is at 109 degrees.
The 'X' formed by where the two lines cross each other marks your location. If they don't quite reach each other, as in our example on the left, don't worry. Simply extend them, slightly, with the scale edge of your compass. However, try to avoid using features so far apart that they require excessive extension of your lines. The more you have to extend the lines, the more error you introduce. If you must extend a line more than about half a compass length, try to use a longer straight edge to extend it from the original line. Then, verify the extension has retained the correct azimuth using your compass.
Choosing terrain features that create intersections as close to 90 degree right angles as possible will give you the most accurate result. However, in most situations an ideal right angle will not present itself and you will have to compromise somewhat. Just know that the further from 90 degrees you get, the less precise the result will be. You can plot a third line from another feature to enhance the accuracy, though it is not strictly necessary.
The third line can also be useful to verify the accuracy of all your readings. Because if the azimuth of any of the three points are improperly measured, you've mis-identified any of the terrain features on the map or, the map has been bumped out of alignment during measurements, the three lines will not intersect properly. Below, a third line was added from a hill at a bearing of 34 degrees. This process is called 'Triangulation'.
When all the measurements are perfectly accurate, all three lines intersect as in the image on the left. Due to slight inaccuracies in even high quality compasses as well as a person's measurements themselves, the lines typically form a small triangle around your position. In the image on the right, a 10 degree azimuth error was intentionally introduced to the first line (the one coming from the upper left side of the image). This large triangle indicates there's an error and a double check of all your data is in order. If the map is still aligned north and all the azimuth measurements still check out, then you've likely misidentified one of the terrain features on the map. Or, something might be wrong with your compass that is causing the inaccuracy.
A final note on measurement errors. In the above left image, the compass is reading an azimuth of 34 degrees as mentioned above. Yet, because the image is taken from an angle, it appears to read more like 30. This is a good demonstration of the importance of always reading the dial looking from directly above, because even the fairly slight angles can change your perceived reading by several degrees.